For such applications, it is essential to know the position or variation in position of the interface between the two fluids in order to be able to determine the depth the excavation has reached (the excavation generally proceeds from the bottom toward the surface) and so monitor the development of the cavity.
It will be noted that the fluids present in the cavity are often fuel oil (in the top portion) and brine (in the lower portion), with the brine formed from water injected under appropriate pressure.
Similar monitoring processes already exist in which the fluids in question move through one and/or another of two pipes leading to and/or from the cavity. The two pipes generally extend concentrically into the cavity, one inside the other.
The probe, in such known means, is usually connected to a stationary exterior point by a cable link from which it is suspended. Said probe generally comprises at least one signal-emitting source and at least one detector-receiver.
With such means, readings of the position of the interface are usually done by logging, which involves:
The use of a complete on-site logging unit (mobile laboratory and specialized team) for each movement in the position of the fluids if the probe is to be accurately positioned at the spot in which the measurements are to be made;
The permanent presence of probe and cable inside the center pipe.
In practice, such an arrangement of the probe within the central pipe may prove fatal. This is so particularly when one seeks to monitor a salt cavity formed by a technique known as reverse leaching, which is in itself known.
In such applications, saturated brine to be removed moves through the inner pipe. The presence of measuring instruments inside the pipe impedes proper flow. The resulting losses of head are too high. In addition, probe and cable tend to wear out prematurely because of the salt in the brine.